This application relates to a method for making molded polycarbonate articles with improved color.
Polycarbonate is a thermoplastic that has excellent mechanical properties such as impact resistance, heat resistance and transparency. Polycarbonates are widely used in applications ranging from football helmets to automobile parts to transparent security windows. More recently, polycarbonates have also proven to be the material of choice for optical media applications such as optical discs, for example compact discs (CD) and digital versatile discs (DVD). Conventional polycarbonates are usually produced by (1) an interfacial polymerization, in which a precursor compound such as bisphenol A (BPA) is reacted directly with phosgene or by (2) a melt polymerization process in which a dihydroxy compound such as BPA is transesterified with a diaryl carbonate such as diphenyl carbonate (DPC) or an activated diaryl carbonate such as bismethylsalicylcarbonate (BMSC). For many applications, there has been a need for materials possessing the fundamental characteristics of transparency and toughness inherent in BPA polycarbonate but possessing, in addition, certain improvements in physical properties relative those possessed by bisphenol A polycarbonate (BPA-PC), for example birefringence. For some applications improved chemical resistance relative to BPA polycarbonate is required, for example in certain medical and automotive applications. Copolycarbonates are materials frequently possessing the fundamental traits of BPA polycarbonate, transparency and toughness, but in certain instances also possessing improved performance characteristics for a given application relative to BPA polycarbonate.
Color can be generated in polycarbonates or in copolycarbonates which reduces the transparent quality of resulting articles formed from such. The quality of the starting materials has a large effect on the color of the final polycarbonate. Further, the polymerization and molding conditions used to produce the polycarbonate and molded articles may lead to the formation of side reactions within the polycarbonate leading to color formation therein. Greater color formation occurs when the molding conditions are more abusive, and thus to provide the greatest range of manufacturing versatility, methods for controlling color formation during molding are desired.
The present invention relates to a method for improving the color of molded polycarbonate articles that make use of polycarbonate made by melt transesterification with an activated diaryl carbonate, particularly ester-substituted diaryl carbonates such as bismethylsalicylcarbonate (BMSC). Such processes are described in the art, for example in U.S. patent application Ser. Nos. 4,323,668, 5,696,222, 6,300,459, 6,420,512, 6,790,929, 6,870,025 and 7,034,099, all of which are incorporated herein by reference. While manufacture of polycarbonate using activated diaryl carbonates offers many benefits, one downside of the approach is the formation of product with greater color than a comparable product made using diphenyl carbonate.